1. Field of the Invention
The present invention relates to a jet-propelled boat in which pressure is applied to water by a jet propulsion unit driven by a two-stroke cycle engine to jet water for propelling a hull of the boat. More particularly, to a valve driving mechanism for driving an exhaust control valve installed on the two-stroke cycle engine.
2. Description of Related Art
A jet-propelled boat as mentioned in, for example, JP-A No. 62-35013 and JP-U No. 5-27270, is a small ship in which an operator straddles a saddle type seat. Such a jet-propelled boat provided with a saddle type seat is used as a recreational boat or a rescue boat owing to its sharp steering characteristic and high mobility. Jet propulsion units for ajet-propelled boat mentioned in, for example, JP-A Nos. 62-59194 and 5-246385 have an impeller driven for rotation by an engine, water is sucked from outside the boat, pressurized and jetted by the rotation of the impeller to propel the boat. The jet propulsion unit mentioned in the former cited reference takes pressurized water from the downstream side of the impeller and uses the water as cooling water for cooling the engine. The jet propulsion unit mentioned in the latter cited reference takes pressurized water from the downstream side of the impeller and uses the water as cooling water for cooling a bearing unit supporting the impeller.
As is generally known, a two-stroke cycle engine has a relatively simple construction and is capable of providing a high power output for its size. As mentioned in, for example, JP-A Nos. 56-56915 and 56-60817, such a two-stroke cycle engine is provided with an exhaust control valve for varying exhaust timing on the inner surface of an exhaust passage at a position near the upper edge of an exhaust port. The exhaust control valve is opened to advance the time to start discharging an exhaust gas so that scavenging is promoted while the engine is operating at a high engine speed and the exhaust control valve is half closed to delay the time to start discharging an exhaust gas so that the blowby of a new charge is prevented while the engine is operating at a low engine speed to enhance the output power of the engine and to improve the fuel consumption of the engine.
A two-stroke cycle engine may change the exhaust timing thereof by driving an exhaust control valve by the pressure of engine cooling water increasing with the increase of engine speed.
In two-stroke cycle engines disclosed in JP-A Nos. 7-71279 and 7-180556, an exhaust control valve for opening and closing an exhaust port is disposed on the inner surface of an exhaust passage at a position near the upper edge of the exhaust port, the exhaust port is substantially fullly closed by the exhaust control valve while the two-stroke cycle engine is operating in a low-speed, low-load operation to control the pressure in the cylinder properly, to activate a new charge charged into the combustion chamber by the thermal energy of a combustion gas remaining in the combustion chamber and to make self-ignition combustion in an activation heat atmosphere so that the fuel consumption of the engine is improved and the exhaust gas is purified. Traveling speed, and engine speed and throttle opening representing load on the engine are measured, engine output power is estimated on the basis of a measured traveling speed, a measured engine speed and a measured throttle opening, the exhaust control valve is opened to increase the ratio of exhaust opening while the engine is operating at a high engine speed under a high load, and the exhaust control valve is closed to close the exhaust port while the engine is operating at a low engine speed under a low load.